John Joseph Martin b. 1815 Co. Cork. Ireland

I am looking for any information on a John J. Martin b. 1815 in County Cork, Ireland, he could have married a lady by the name of Mary Ellen McKee in about 1829. Yes, young! But they probably lived with his parents for awhile. However, I haven't found anything on them, no passenger lists to America but have found one census with them in Brooklyn, Kings Co. NY with their children. I know that this family came to Chicago, Cook Co. IL where at least their 4 son's died, however their only daughter died also but I don't have that information either. Anyone have any ideas about going about locating them in Ireland before John and Mary Ellen came to the USA? They should be either in Ireland or England in 1841 and I have tried to find them, but can't for some reason. I understand that they came to the USA in 1854-1856 but haven't found that either. HELP PLEASE??? Melody